1. Field of the Invention
The present invention relates to a vibration type probe structure for measuring the surface potential of a surface of a sample in a noncontact manner and a surface potential measuring system using the same.
2. Description of the Related Art
As a conventional system for measuring charges on a surface of a sample, a noncontact type surface potential measuring system shown in FIG. 1 is known. In the system shown in FIG. 1, a measurement hole 100a is formed in the bottom of a shield case 100, and a flat sample 102 having a surface potential E is placed under the measurement hole 100a. A flat probe electrode 101 is arranged in the shield case 100 so as to oppose the sample 102 at a predetermined distance. A sector 103 as a shutter is arranged between the measurement hole 100a and the probe electrode 101. The sector 103 is connected to a solenoid 104 for driving the sector 103, a solenoid driver 105 for energizing the solenoid 104, and an oscillator 106 for generating an oscillation signal. A signal generated by the oscillator 106 is amplified by the solenoid driver 105 and is supplied to the solenoid 104. The solenoid 104 then drives the sector 103. The sector 03 is moved parallel to the measurement hole 100a to open and close it. In addition, an amplifier 107 and a synchronous detection circuit 108 are connected to the probe electrode 101 through resistors R1 and R2 and a capacitor C.
In this conventional surface potential measuring system, some of lines of electric force extending from the surface of the sample 102 reach a surface of the probe electrode 101 through the measurement hole 100a, and their amount .phi. is changed at a constant period upon an opening/closing operation of the sector 103. Therefore, a current proportional to d.phi./dt flows through the load of the probe electrode 101, and an AC voltage e having a predetermined period is generated across the two ends of the capacitor R1 upon the opening/closing operation of the sector 103. The AC voltage e is proportional to the surface potential E of the sample 102 provided that the amplitude and frequency of the sector 103 and the distance from the probe electrode 101 to the sample 102 are constant.
The AC voltage e detected by the probe electrode 101 has a much smaller level than the surface potential E of the sample 102. For this reason, the AC voltage e is amplified by the amplifier 107 to a predetermined level and is converted into a DC voltage by the synchronous detection circuit 108 in synchronism with the oscillation frequency of the oscillator 106, i.e., the opening/closing operation of the sector 103 so as to be output as a measurement signal.
In this conventional surface potential measuring system, in order to obtain a measurement output proportional to the surface potential E of the sample 102, a signal to be detected by the probe electrode 101 must be a signal which is not much influenced by external noise and has a predetermined level or more.
For this purpose, the surface area of the probe electrode 101 must be increased, and the measurement hole 100a must be formed to have a predetermined size or more (generally, 3 mm square or more). In the conventional surface potential measuring system, therefore, the surface potential E of the sample 102 cannot be measured unless the sample 102 has an area of about 10 mm.sup.2 or more. That is, a surface potential in a small area smaller than an area of 10 mm.sup.2 cannot be measured. In addition, since the selector 103 as a shutter is arranged between the probe electrode 101 and the sample 102, the distance between the probe electrode 101 and the sample 102 is undesirably increased, resulting in poor detection sensitivity.
Furthermore, in the conventional noncontact type surface potential measuring system, a detection current value error is caused upon measurement due to temperature drift and the like. For example, the bias current of a detection amplifier is changed with a change in temperature, whereas the permittivity of a capacitance determined by the sample and the probe structure is changed with a change in humidity. Therefore, such an error must be corrected.
For this purpose, in the conventional system, prior to measurement, a reference voltage is applied to a conductive plate arranged in place of the sample 102. The reference voltage is changed, and a value measured by the surface potential measuring system at this time is calibrated. Thereafter, the sample 102 is placed under the system so as to measure its surface potential.
In such a method, however, a measurement value under the conditions of calibration is changed over time due to drift such as temperature drift. In order to perform high-precision measurement, therefore, measurement must be quickly started and completed after calibration so as to prevent the influences of temperature drift and the like. However, if, for example, a transfer drum used for a copying machine or a transfer disk used for a system for measuring the surface potential of a disk is a sample, since it has a large measurement area, a long measurement time is required. As a result, a measurement error caused by temperature drift and the like cannot be neglected, and stable, high-precision measurement cannot be performed.